Several publications, including Japanese Patent Application Laid-Open (kokai) No. 11-199320, disclose that when an oxide such as Al2O3 or MnO2 is incorporated into a CaTiO3-based composition having a perovskite-type crystal structure and serving as the primary component of a dielectric ceramic composition, there can be obtained a dielectric ceramic composition which exhibits, in microwave ranges, practically sufficient relative dielectric constant (hereinafter referred to as “∈r”) and unloaded quality factor (hereinafter referred to as “Qu”), and small absolute value of the temperature coefficient of resonance frequency (hereinafter the temperature coefficient will be referred to as “τf”). The dielectric ceramic composition exhibits excellent dielectric characteristics; specifically a Qu of 3,500 to 4,200 and a τf of 0 to +8.0 ppm/° C.
However, in recent years, the frequency range within which dielectric ceramic compositions are employed has been expanded from microwave ranges to sub-millimeter wave ranges, and thus a dielectric ceramic composition exhibiting higher Qu has been demanded. Particularly when a dielectric ceramic composition is employed for forming a large-sized dielectric resonator, if the entirety of the ceramic composition is not sufficiently sintered and thus uniformity is not attained, the resultant dielectric resonator involves problems such as lowering of Qu. Therefore, demand has arisen for a dielectric ceramic composition which exhibits ∈r and Qu higher than those of a conventional dielectric ceramic composition and small absolute value of τf, and which, when sintered sufficiently at its interior and exterior regions, enables production of a large-sized dielectric resonator that consistently exhibits high Qu. Also, demand has arisen for a dielectric resonator produced from such a dielectric ceramic composition.
In view of the foregoing, an object of the present invention is to provide a dielectric ceramic composition which exhibit practically sufficient ∈r and Qu and which attains smaller absolute value of τf. Another object of the present invention is to provide a dielectric resonator product produced from the dielectric ceramic composition exhibiting excellent dielectric characteristics, which exhibits high Qu even when of large size.